Saturday, April 20, 2013

Gold Sample Values and Placer Evaluation (Part 1)

(What works is whatever helps you get more gold.)

A while back one of you readers out there commented that you'd like to know how to calculate gold values contained in drill (core) samples. I'm not sure if I can answer that question specifically to your satisfaction, but I'll give it a good try try while passing additional info along that may prove valuable to all.

(Note: In this series of posts I'll be speaking about placer gold values only since the great majority of you are placer and not hard rock miners. My apologies to you hard rock gold slight intended.)

Gold Values

Once again, in the United States the standard volume measurement for gold-bearing placer material is expressed in terms of cubic yards or bank cubic yards for you purists. Out there in the rest of this big, wide world gold values are typically measured in milligrams of gold per unit of volume. In both instances, the fineness of the gold in a given placer and the price of gold at the time of evaluation are used to come up with what's known as a placer "valuation factor."

OK you math freaks, here's a simple placer test pit or sample hole valuation equation that's been around for quite a while:

 Va = Sum of material (d x  V)
               Sum of d

Where Va = avg. value of block, triangle, hole in $$$ per cubic yard.

d = depth of each block, triangle, or hole in feet.

V = value of each prospect block, triangle, hole in $$$ per cubic yard

(Note: I'll discuss blocks and triangles in my next post.)

Sample Splits

Here's where things get a bit confusing. You heard me mention in an earlier post about samples being split into quarters, eights, and so on for assaying. I'm not sure, but I may not have qualified that for you then so let me do it now. It's not uncommon to split hard-rock samples before assaying, but  placer sample splits are a no-no according to some.

Gold Concentrates
Gold Pans
Gold Prospecting Books
Here's where I might step into a pile of doggie doo-doo again. Many "experts" these days say you should never split samples regardless whether they're hard rock or placer. No way, no how. NEVER. The idea here is that when sample volumes are reduced you can risk ending up with false or erratic's the old "garbage in, garbage out" philosophy.

God forbid I should challenge all those "experts" out there, but the old timers who mentored me when I was still an arrogant young pup who was greener than green taught me otherwise. In fact, they were adamant on that particular point.

Who's Right or Wrong?

Now, these "gentleman" (i.e., no-BS hard asses) had seen and done it all during their lifetimes and they knew their gold mining, both hard rock and placer. They taught me that reducing sample sizes before assay or valuation was a perfectly right-minded way of going about things when it comes to sampling and would give a more accurate idea of the gold values to be expected. This was supposed to be even more true when you were sampling low-grade ground containing widely disseminated gold.

(The old timers were tough customers, but great mentors and teachers.)

Who's right and who's wrong here? That'll be for you to decide. I do know this...I've never had a self-proclaimed "expert" ever put my nose right into the gold, but my mentors did many times. No disrespect to all those PhDs out there, but I'll put my money on the old timers who lived and breathed gold and gold mining and taught me all I know.

There's more to come, so stay tuned.

To all of you who've supported this blog and myself over the years, especially during the low points, this one's for you! Thanks.

If you liked this post, you may want to read: "Gold Sampling: a 3-Phase Plan of Attack (Conclusion)"

(c)  Jim Rocha (J.R.)  2013

Questions? E-mail me at


  1. Ya know...I'm finding out theres a lot of folks out there that are way more brainy than me....I can dig a hole....but thats 'bout it.

  2. Well my friend, I've never been a math wizard myself but I am good at certain calculations. Eventually either way it's about digging! J.R.